When converting rotate transform functions `sin` and `cos` can sometimes
be inaccurate. To avoid these inaccuracies we:
- Mod the angle to minimise inaccuracies in the first place.
- Discard tiny (smaller than epsilon) values returned by `sin` and
`cos` as inaccuracies.
This is in line with other browsers (e.g. Gecko and WebKit).
This patch just adds the new root paintable and updates the tests
expectations. The next patch will move painting logic from the layout
viewport to the paint viewport.
